Question

Draw a cross-sectional sketch of a normal fault. Label the hanging wall and the footwall. Use your sketch to explain how a normal fault accommodates crustal extension. Sketch a re verse fault and show how it accommodates crustal shortening.

Short answer

Normal faults show crustal extension, while reverse faults show crustal shortening.

Step-by-step solution

Understanding the Components of a Normal Fault

A normal fault occurs when rock slabs are pulled apart due to tectonic forces. It features two main components: the hanging wall and the footwall. The hanging wall is the block of rock that lies above the fault plane, while the footwall is below it.

Sketching a Normal Fault

Draw a diagonal line from the bottom left to the top right to represent the fault plane. Label the top block as the 'Hanging Wall' and the bottom block as the 'Footwall'. Indicate movement by drawing an arrow pointing downwards on the hanging wall side, and an arrow pointing upwards on the footwall side.

Explaining Crustal Extension from Normal Fault

In a normal fault, the hanging wall moves downward relative to the footwall, creating space and thinning the Earth's crust. This motion reflects tectonic forces pulling blocks apart, leading to crustal extension, typically found in divergent plate boundaries.

Understanding the Components of a Reverse Fault

In a reverse fault, tectonic forces push rock slabs together, with the hanging wall moving upwards relative to the footwall. This kind of fault results from compressive stress, opposite to a normal fault.

Sketching a Reverse Fault

Draw a diagonal line similar to the normal fault but indicate an upward movement for the hanging wall and a downward shift for the footwall. Label the top block as 'Hanging Wall' and the bottom block as 'Footwall', showing how they push together.

Explaining Crustal Shortening from Reverse Fault

In a reverse fault, the upward movement of the hanging wall over the footwall causes the crust to shorten and thicken. This motion is characteristic of convergent plate boundaries where tectonic plates collide.

Key concepts

Normal Fault

A normal fault is a type of fault where the ground is pulled apart by tectonic forces. Imagine the Earth's crust being like a stretched elastic band. When the crust is under tension, it causes sections of rock to break and move along a fault line.

This breaking creates two main parts: the **hanging wall** (the section of rock above the fault) and the **footwall** (the part below the fault). In a normal fault, the hanging wall moves downward in relation to the footwall.

This downward motion is caused by forces pulling the crust apart, which is why we say it 'accommodates crustal extension.' As a result, the Earth's crust becomes thinner in the area of the fault. You can think of this process as stretching and thinning a piece of dough by pulling it.

Normal faults are common in regions where tectonic plates are moving away from each other, known as divergent plate boundaries.

Reverse Fault

A reverse fault occurs due to compressive forces—a bit like giving the Earth's crust a big squeeze. In a reverse fault, the **hanging wall** moves upward compared to the **footwall**. This is the opposite movement to a normal fault.

Think of it like two cars in a collision; they push into each other, and one car may even go over the other. This interaction causes the Earth's crust to become shorter and thicker, a process known as 'crustal shortening'.

Reverse faults typically form in regions where plates are converging, or moving towards each other, such as at **convergent plate boundaries**. These areas face intense pressure that can lead to the development of large mountain ranges over time.

Crustal Deformation

Crustal deformation refers to changes in the shape, position, or orientation of rocks within the Earth's crust. It occurs due to the powerful forces acting upon tectonic plates.

There are two primary types of deformation that affect the crust:

• **Crustal Extension**: This happens when the Earth's crust is stretched and thinned, such as with normal faults.
• **Crustal Shortening**: This occurs when sections of the crust are compressed and thickened, as seen in reverse faults.

These processes are essential in shaping the Earth's surface, leading to formations like mountains, valleys, and basins. Earth's dynamic nature is a result of these deformations, forming diverse landscapes as tectonic forces continually reshape our planet's surface.